1. Field of the Invention
The invention generally relates to an apparatus for measuring the position of a moving member relative to a stationary member and, in particular, a scale on a rod of a linear motion cylinder (having a rod, housing, gland and driving element such as a piston or rotor) and a detector on the cylinder housing for detecting the scale to indicate the position of the rod relative to the housing, thus detecting the length of the cylinder.
2. Description of the Prior Art
Computerized motion control of actuated cylinders requires rod position and velocity measurements. These measurements must be accurate, repeatable and have sufficient resolution for the particular application. Many applications also require the measurement to be cost effective, easily redundant, immune to mechanical noise and insensitive to debris. The problems with measuring the position of a rod of a linear motion cylinder are particularly acute. Current hydraulic cylinder measurement practice uses either string encoders with multi-turn potentiometers, linear potentiometers or magnetostrictive devices. All of these approaches can be problematic for commodity applications. Magneto-strictive devices are expensive and require either hollow rods or extensive external mountings. On the other hand, string encoders are less expensive but are frail, are subject to animal damage, and are degraded by mechanical noise. Linear potentiometers are length limited and subject to mechanical noise.